Method of forming a gate contact

ABSTRACT

A method is provided for forming a gate contact for a compound semiconductor device. The gate contact is formed from a gate contact portion and a top or wing contact portion. The method allows for the tunablity of the size of the wing contact portion, while retaining the size of the gate contact portion based on a desired operational frequency. This is accomplished by providing for one or more additional conductive material processes on the wing contact portion to increase the cross-sectional area of the wing contact portion reducing the gate resistance, while maintaing the length of the gate contact portion to maintain the operating frequency of the device.

TECHNICAL FIELD

The present invention relates to semiconductors, and more particularlyto a method of forming a gate contact.

BACKGROUND

Group III-V semiconductor transistors, such as high electron mobilitytransistors (HEMTs), require smaller gate contact lengths (Lg) (lessthan 40 nm) and low gate contact resistance (Rg) for high frequencyoperation. However the smaller gate contact size normally associateswith higher gate resistance (Rg) due to the smaller physical dimensionsof gate contact length. Historically, fabrication of a T-shaped gatecontact (T-gate contact) approach has been implemented in the industryfor group III to V semiconductor transistors. The T-shaped gate contactemploys a small gate contact portion and a larger top contact portion,referred to as a wing. The purpose of the larger top contact portion(wing) is to increase the cross sectional area of gate (to lower the Rg)while maintaining the smaller gate length (lower Lg). However, when theT-gate is scaled down to smaller gate contact sizes (less than or equalto 40 nm), the wing contact portion size scales down proportionally andhas smaller dimensions.

Some techniques have been employed to form gate contacts with smallergate contact portion size with larger wing contact portions. One of thetechniques employed to form gate contacts with smaller gate contactportion sizes with larger wing contact portions involves using twoseparate exposure/development steps and two separate metalization steps.The gate contact is formed in the first resist exposure, development,and metallization processing step, and the wing contact portion in thesecond resist exposure, development, and metallization processing step.This process would allow the wing to be sized to many selective wingsizes. However the disadvantages are that the registration of the wingcontact portion to the gate contact portion can be misaligned.Furthermore, the increased number of processing steps to achieve theresults will lower the total gate yield when dealing with gate sizes of40 nm or smaller. The number of processing steps will also effectivelyincrease the length of processing time and cost.

Another technique is to use a two step resist exposure process ofexposing and developing a resist of a wing contact portion first andthen exposing and developing a resist of a gate contact portion. Howeverthere can be misalignment of wing contact portion to gate contactportion registration, and the gate size control/uniformity can degradeby wing dose contribution to the gate dose.

A third technique is to use multiple layers of resist, and use a set ofdevelopers. Each developer specifically develops each resist layerindividually. This allows the wing size to be developed for longer timeresulting in a larger wing. However, each of the resist layers needs tobe developed completely, so that the next developer can develop the nextlayer correctly. Any resist not completely developed away will preventthe next developer from opening the resist for the next layer.Additionally, the different resists can form an inter-mixed layer whichcan be difficult to develop out completely by any of the developers.

SUMMARY

In accordance with an aspect of the invention, a method is provided forforming a gate contact for a compound semiconductor device. The methodcomprises depositing a first photoresist material layer over a substrateand depositing a second photoresist material layer over the firstphotoresist material layer. The second photoresist material layer has ahigher sensitivity to a photoresist development process than the firstphotoresist material layer. The method further comprises performing aphotoresist development process to form a first via in the firstphotoresist material layer and a second larger via, overlying the firstvia, in the second photoresist material layer, performing a firstconductive material deposition process to form a gate contact having agate contact portion formed in the first via in contact with thesubstrate and a wing contact portion disposed over and in contact withthe gate contact portion in the second larger via, and stripping aconductive material layer that results from the first conductivematerial deposition process from the second photoresist material layer.The method also comprises performing etching process to removeadditional portions of the second photoresist material layer from thesecond larger via to laterally extend the second larger via, andperforming a second conductive material deposition process to form anouter wing contact portion to increase the cross-sectional area of thewing contact portion, while maintaining the length of the gate contactportion.

In another aspect of the invention, a method for forming a compoundsemiconductor device is provided. The method comprises depositing afirst photoresist material layer over a substrate, and depositing asecond photoresist material layer over the first photoresist materiallayer. The second photoresist material layer has a higher sensitivity toa photoresist development process than the first photoresist materiallayer. The method further comprises performing an electron beamlithography process to form a first via in the first photoresistmaterial layer and a second larger via, overlying the first via, in thesecond photoresist material layer, depositing a first conductivematerial to form a gate contact having a gate contact portion formed inthe first via in contact with the substrate and a wing contact portionlocated over and in contact with the gate contact portion in the secondlarger via, and stripping a conductive material layer that results fromthe depositing the first conductive material from the second photoresistmaterial layer. The method further comprises performing an oxygen O₂plasma process to remove additional portions of the second photoresistmaterial layer from the second larger via to laterally extend the secondlarger via, and evaporating and depositing a second conductive materialto form an outer wing contact portion to increase the cross-sectionalarea of the wing contact portion, while maintaining the length of thegate contact portion, wherein the length of the gate contact portion isselected to provide a desired operating frequency and thecross-sectional area of the wing contact portion is selected to providea desired gate contact resistance without deleterious effects on thedesired operating frequency.

In accordance with yet another aspect of the invention, a compoundsemiconductor device is provided that comprises a substrate and a gatecontact having a gate contact portion in contact with the substrate anda wing contact portion disposed over and in contact with the gatecontact portion. The wing contact portion has an inner wing contactportion and an outer wing portion, wherein the outer wing contactportion increases the cross-sectional area of the wing contact portion,while maintaining the length of the gate contact portion. The length ofthe gate contact portion is selected to provide a desired operatingfrequency and the cross-sectional area of the wing contact portion isselected to provide a desired gate contact resistance withoutdeleterious effects on the desired operating frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of a compoundsemiconductor structure in accordance with an aspect of the presentinvention.

FIG. 2 is a chart of gate resistance for standard wing base line versuslarger wing base line for gate sizes of 40 nm or less.

FIG. 3 is a schematic cross-sectional illustration of first and secondphotoresist material layers overlying a substrate for forming inaccordance with an aspect of the present invention.

FIG. 4 is a schematic cross-sectional illustration of the structure ofFIG. 3 undergoing a photoresist development process in accordance withan aspect of the present invention.

FIG. 5 is a schematic cross-sectional illustration of the structure ofFIG. 4 after undergoing the photoresist development process inaccordance with an aspect of the present invention.

FIG. 6 is a schematic cross-sectional illustration of the structure ofFIG. 5 undergoing a first conductive material deposition process inaccordance with an aspect of the present invention.

FIG. 7 is a schematic cross-sectional illustration of the structure ofFIG. 6 after undergoing the first conductive material deposition processin accordance with an aspect of the present invention.

FIG. 8 is a schematic cross-sectional illustration of the structure ofFIG. 7 undergoing an isotropic oxygen O₂ plasma process in accordancewith an aspect of the present invention.

FIG. 9 is a schematic cross-sectional illustration of the structure ofFIG. 8 after undergoing the isotropic oxygen O₂ plasma process inaccordance with an aspect of the present invention.

FIG. 10 is a schematic cross-sectional illustration of the structure ofFIG. 9 undergoing a second conductive material deposition process inaccordance with an aspect of the present invention.

FIG. 11 is a schematic cross-sectional illustration of the structure ofFIG. 10 after undergoing the second conductive material depositionprocess in accordance with an aspect of the present invention.

FIG. 12 is a schematic cross-sectional illustration of the structure ofFIG. 11 after stripping of a conductive material layer from the secondphotoresist material layer in accordance with an aspect of the presentinvention.

DETAILED DESCRIPTION

The present disclosure provides for a method of forming a gate contactfor a compound semiconductor device, such as a high electron mobilitytransistor (HEMT). A compound semiconductor device includes twodifferent atomic elements in each layer of the semiconductor device. Forexample, the semiconductor device can have semiconductor layers formedfrom Group III-V semiconductor materials, such as Gallium Nitride (GaN),Gallium Arsenide (GaAs), Indium Phosphide (InP) or other compoundsemiconductor. The gate contact is formed from a gate contact portionand a top or wing contact portion. The method allows for the tunablityof the size of the wing contact portion, while retaining the size of thegate contact portion based on a desired operational frequency. This isaccomplished by providing for one or more additional conductive materialprocesses on the wing contact portion to increase the cross-sectionalarea of the wing contact portion reducing the gate resistance, whilemaintaing the length of the gate contact portion to maintain theoperating frequency of the device.

FIG. 1 is a schematic cross-sectional illustration of a portion of acompound semiconductor structure 10 in accordance with the presentinvention. The compound semiconductor structure 10 includes a gatecontact 14 that overlies a substrate 12. The substrate 12 can be adielectric layer overlying a channel region (not shown) of a transistor.The transistor can be, for example, a high electron mobility transistor(HEMT) formed from various compound semiconductor material layers. Thegate contact 14 has a generally pine tree shape or “T” shape to minimizeresistance which provides high device operating performance. The gatecontact 12 can be formed from one or more conductive materials, such asgold, aluminum, copper, platinum, or other conductive material layer.

The gate contact 12 is formed of a gate contact portion 16 that is incontact with the substrate 12 and a top or wing contact portion 18 incontact with and overlying the gate contact portion 16. The gate contactportion 16 has a length that is selected based on a desired operatingfrequency, while the wing contact portion 18 is selected to have across-sectional area that minimizes the resistance of the gate contact14, which also contributes to high operating frequency. The wing contactportion 18 includes an inner wing contact portion 20 and an outer wingcontact portion 22. The inner wing contact portion 20 and the outercontact portion 22 were formed from different conductive materialdeposition processes to increase the cross-sectional area of the wingcontact portion 14 without increasing the length of the gate contactportion 16, as will be described below.

It is to be appreciated that when the gate contact is scaled down tosmaller gate size (≦40 nm), the wing size scales down proportionally andhas smaller dimensions. The wing size can decrease to as much as 30% ofthe original wing size when the gate size is in this range. Byincreasing the wing size using the disclosed methodology, the gateresistance can be reduced as shown in chart 30 of FIG. 2.

Turning now to FIGS. 3-13, process blocks in connection with fabricationof a portion of compound semiconductor structure (e.g., HEMT) inaccordance with an aspect of the present invention are described. Afirst photoresist material layer 54 is deposited over a substrate 52(e.g., compound semiconductor substrate, dielectric layer) and a secondphotoresist material layer 56 is deposited over the first photoresistmaterial layer 54. The first photoresist material layer 54 and thesecond photoresist material layer 56 may be deposited using any suitablemeans. For example, the first photoresist material layer 54 may bedeposited over the substrate 52 utilizing spin-coating or spin castingdeposition techniques. Once the first photoresist material layer 54dries, then the second photoresist material layer 56 may be depositedover the first photoresist material layer 54 utilizing spin-coating orspin casting deposition techniques. The second photoresist materiallayer 56 is selected to have a higher sensitivity to a photoresistdevelopment process than the first photoresist material layer 54, suchthat the second photoresist material layer 56 provides a larger via thanthe first photoresist material layer 54 upon completion of thephotoresist development process.

FIG. 4 illustrates the structure of FIG. 3 undergoing a photoresistdevelopment process 100 in which an exposing source (such as opticallight, x-rays, or an electron beam) illuminates selected areas of thefirst and second photoresist material layers 54 and 56 through anintervening mask, for a particular pattern, to become either more orless soluble (depending on the coating) in a particular solventdeveloper. Next, as represented in FIG. 5, solvent developer is appliedto the exposed first and second photoresist material layers 54 and 56 toopen a first via 58 in the first photoresist material layer 54 and asecond larger via 60 in the second photoresist material layer 56 thatoverlies the first via 58. In one aspect of the invention, thephotoresist development process employs electron beam lithography.However, any suitable photolithographic techniques can be performed toform the patterned first and second photoresist material layers 54 and56.

FIG. 6 illustrates the structure of FIG. 5 undergoing a first conductivematerial deposition process 110. Any suitable technique for depositingthe conductive material may be employed such as metal evaporation,sputter evaporation, plating, Low Pressure Chemical Vapor Deposition(LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic LayerDeposition (ALD), sputtering or spin on techniques. The conductivematerial can be one or more conductive materials and/or conductivematerial layers, such as gold, aluminum, copper, platinum, or otherconductive materials.

FIG. 7 illustrates the structure of FIG. 6 after the conductive materialdeposition process 110 in which a gate contact 61 has been formed. Thegate contact 61 has a generally pine tree shape or “T” shape with a gatecontact portion 62 that is in contact with the substrate 52 and a top orwing contact portion 64 overlying the gate contact portion 62.Additionally, a conductive material layer 66 has formed overlying thesecond photoresist material layer 56 as a result of the conductivematerial layer process 110. The conductive material layer 66 includesportions that overly the second larger via 60, which prohibits furtherforming of the cross-sectional area of the wing contact portion 64.

Next, the conductive material layer 66 is stripped from the secondphotoresist material layer 56 and the resultant structure is subjectedto an isotropic oxygen O₂ plasma process 120, as illustrated in FIG. 8.The isotropic oxygen O₂ plasma process 120 removes additional portion ofthe second photoresist material layer 56 and laterally widens the secondlarger via 60. The amount of lateral widen depends on the length of timeof the plasma process and/or the plasma rate. It is to be appreciatedthat other etching techniques could be employed to laterally widen thesecond larger via 60.

It was discovered that the stripping of the conductive material layer 66results in the top surface of the second photoresist material layer 56becoming more resistant to the isotropic plasma process 120 than thephotoresist material in the second larger via 60. Therefore, thephotoresist material in the second larger via 60 etches at a faster ratethan the top surface of the second photoresist material layer 56. FIG. 9illustrates the resultant structure of FIG. 8 with a laterally extendedsecond via 68.

FIG. 10 illustrates the structure of FIG. 9 undergoing a secondconductive material deposition process 130. The second conductivematerial deposition process 130 can be a non-conformal conductivematerial deposition and employ a conductive material evaporator whichdeposits conductive material only on the top of the wing contact portion64. The resultant structure is illustrated in FIG. 11, where an outerwing contact portion 72 is formed over the wing contact portion 64,which is now an inner wing contact portion. Again, a conductive materiallayer 74 has formed overlying the second photoresist material layer 56as a result of the conductive material layer process 130. Next, theconductive material layer 74 is stripped from the second photoresistmaterial layer 56 and the resultant structure is illustrated in FIG. 12.

The process can be repeated to form additional outer wing contactportions to increase the cross-sectional area of the wing contactportion 64 within practical limitations. The first and secondphotoresist material layers 54 and 56 illustrated in FIG. 12 can bestripped employing a wet chemical strip to provide the resultantstructure illustrated in FIG. 1.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications, and variations.

What is claimed is:
 1. A method for forming a gate contact for acompound semiconductor device, the method comprising: depositing a firstphotoresist material layer over a substrate; depositing a secondphotoresist material layer over the first photoresist material layer,the second photoresist material layer having a higher sensitivity to aphotoresist development process than the first photoresist materiallayer; performing a photoresist development process to form a first viain the first photoresist material layer and a second larger via,overlying the first via, in the second photoresist material layer;performing a first conductive material deposition process to form a gatecontact having a gate contact portion formed in the first via in contactwith the substrate and a wing contact portion disposed over and incontact with the gate contact portion in the second larger via;stripping a conductive material layer that results from the firstconductive material deposition process from the second photoresistmaterial layer; performing an etch process, after the stripping of theconductive material, to remove additional portions of the secondphotoresist material layer from the second larger via to laterallyextend the second larger via; and performing a second conductivematerial deposition process, after the performing of the etch process,to form an outer wing contact portion to increase the cross-sectionalarea of the wing contact portion, while maintaining the length of thegate contact portion.
 2. The method of claim 1, wherein the photoresistdevelopment process employs electron beam lithography.
 3. The method ofclaim 1, wherein the gate contact comprises one of or is formed from oneor more layers of gold, aluminum, copper, platinum, titanium, andtungsten.
 4. The method of claim of 1, wherein the substrate is abarrier layer of a transistor.
 5. The method of claim 4, wherein thetransistor is a high electron mobility transistor (HEMT).
 6. The methodof claim 1, wherein the gate contact has a general pine tree shape. 7.The method of claim 1, wherein performing a second conductive materialdeposition process comprises performing a non-conformal conductivematerial deposition process employing a conductive material evaporator.8. The method of claim 1, further comprising stripping a conductivematerial layer that results from the second conductive materialdeposition process, and stripping the first and second photoresistmaterial layers employing a wet chemical strip.
 9. The method of claim1, wherein the length of the gate contact portion is selected to providea desired operating frequency and the cross-sectional area of the wingcontact portion is selected to provide a desired gate contact resistanceand to maintain the desired operating frequency.
 10. The method of claim1, further comprising repeating: stripping a conductive material layerformed by the second conductive material deposition process, performingan oxygen O₂ plasma process after each of the stripping, and performinga third conductive material deposition process after the performing ofthe oxygen O₂ plasma process to form additional outer wing contactportion.
 11. A method for forming a compound semiconductor device, themethod comprising: depositing a first photoresist material layer over asubstrate; depositing a second photoresist material layer over the firstphotoresist material layer, the second photoresist material layer havinga higher sensitivity to a photoresist development process than the firstphotoresist material layer; performing an electron beam lithographyprocess to form a first via in the first photoresist material layer anda second larger via, overlying the first via, in the second photoresistmaterial layer; depositing a first conductive material to form a gatecontact having a gate contact portion formed in the first via in contactwith the substrate and a wing contact portion located over and incontact with the gate contact portion in the second larger via;stripping a conductive material layer that results from the depositingthe first conductive material from the second photoresist materiallayer; performing an oxygen O₂ plasma process, after the stripping ofthe conductive material, to remove additional portions of the secondphotoresist material layer from the second larger via to laterallyextend the second larger via; and evaporating and depositing, after theperforming of the oxygen O₂ plasma process, a second conductive materialto form an outer wing contact portion to increase the cross-sectionalarea of the wing contact portion, while maintaining the length of thegate contact portion, wherein the length of the gate contact portion isselected to provide a desired operating frequency and thecross-sectional area of the wing contact portion is selected to providea desired gate contact resistance and to maintain the desired operatingfrequency.
 12. The method of claim 11, wherein the gate contactcomprises one of or is formed from one or more layers of gold, aluminum,copper, and platinum.
 13. The method of claim 11, wherein the compoundsemiconductor device is a high electron mobility transistor (HEMT) andthe substrate is a barrier layer of the HEMT.
 14. The method of claim11, wherein the gate contact has a general pine tree shape.
 15. Themethod of claim 11, further comprising stripping a conductive materiallayer that results from the second conductive material depositionprocess, and stripping the first and second photoresist material layersemploying a wet chemical strip.
 16. The method of claim 11, furthercomprising repeating: stripping a conductive material layer formed bythe evaporating and depositing of the second conductive material,performing an additional oxygen O₂ plasma process after each of thestripping, and evaporating and depositing a third conductive materialafter the performing of the additional oxygen O₂ plasma process to forman additional outer wing contact portion.